In order to regulate or control a rotating-field machine, it is advantageous to control the stator current as a function of the respective direction of the magnetic flux passing through the machine. As a result, the component of the machine current that is parallel to the flux is used to set the desired field strength, and the component of the machine current at right angles (i.e. perpendicular) to the flux is used to set the torque or the speed. Such field-oriented operation of the rotating-field machine thus requires a control element for the stator current and a control device which is connected upstream of the stator current control element. The control device receives information about the direction of the flux. This information may be supplied from a machine model which, as an electrical model of the rotating-field machine, uses operating data to determine the flux direction of a simulated modelled flux. If in each case one nominal (i.e. command) value for the field-forming (field-producing) component (parallel to the flux) and for the torque-forming (field-producing) component (perpendicular to the flux) of the machine stator current is now predetermined (field-oriented command values), then the control device can regulate or control the machine current to a stator-oriented stator current vector composed of the two command values with respect to the flux direction of the modelled flux.
In order to execute the control algorithms for dynamically high-quality, (i.e., high performance) field-oriented control concepts with an asynchronous machine running at low speeds, a mechanical sensor is required in order to detect the rotor position or the rotor speed. Asynchronous machines can be operated dynamically and with high quality, without any position sensors or tachogenerators, when the speed is higher than a certain minimum value, so that, from the induced voltage, the magnetic flux can be calculated from electrical variables. Until now, it has not been possible to do this in the low speed range. The aim of many research activities has therefore been to replace the mechanical sensor by mathematical models and/or by using physical effects.
In a field-oriented asynchronous machine without any sensors, the "voltage model" (which uses the machine current and stator voltage of the asynchronous machine as state variables to calculate the magnitude and phase of the magnetic flux in this machine) is used as a machine model. The voltage model calculates the flux as an integral of the electromotive force, that is, as an integral of the voltage less the resistive and inductive voltage drops in the stator. However, the voltage model gives an inaccurate signal at low speeds: errors in the estimated stator resistance, DC voltage components which occur as measurement errors in the voltage measurements, and integration errors in the technical integrators lead to incorrect calculations. Consequently, field-oriented operation that uses a voltage model is only possible at relatively high frequencies. In addition, for the electromotive force integrators, the integration constant must be set by predetermining an initial value.
In some cases, the axis of the electromotive force vector may be used as a direction vector instead of the flux direction, this axis being rotated over 90.degree. electric from the field axis in the steady state. Although this allows the integration to be avoided, accurately controlled operation is not possible in this case at low frequencies. Instead of this, another option for detecting the direction vector is required.
A "current model" simulates the processes in the rotor which lead to the formation of the flux from instantaneous values of the current and rotor position. However, to do this a mechanical sensor is required to determine the rotor position. This complexity is admittedly reduced by an incremental rotation sensor or some other sensor that is not position-coded, such as a pure speed measuring device, but the rotor axis must then be detected (located) in some other way when the machine is stationary (stand-still) or running slowly. This location process is often complex, even with position-coded mechanical sensors.
The problem of locating the rotor position thus occurs particularly in the case of rotating-field machines which have pronounced preferred directions in the rotor, such as reluctance machines. This problem can be solved in the case of machines which have their own field winding since this field winding is still energized when it is stationary, (stand-still) and the resulting flux, which then points in the direction of the rotor axis, is calculated from the voltage induced in the stator. However, this option is impossible with permanent magnet excitation and, furthermore, does not overcome the difficulties that occur at low speeds.
For field-oriented operation of asynchronous machines and at low speeds, the voltage model for determining the position of the flux fails, and the current model has the same problems related to finding the rotor position. A complex mechanical sensor thus often appears to be necessary here as well.
International Patent Application WO 92/19038 describes in detail a number of solutions for the problem of finding the rotor flux position in asynchronous machines, synchronous machines, and reluctance machines. These solutions consist of the rotating-field machine being magnetized in its configuration as an asynchronous machine before the start of the measurement and of the reaction of measurement signals taken from the asynchronous machine being measured. The measurement signals are voltage jumps which are generated by the converter and cause current changes which are measured and fed to a computer which determines a complex characteristic variable which is proportional to the quotient of the stator voltage space vector and the time derivative of the stator current space vector. The direction of the voltage space vector is obtained from the known converter drive state, which is designated as a complex characteristic variable in the following text. The magnetic flux is calculated, both the real part and the imaginary part of the complex characteristic variable fluctuating virtually sinusoidally at twice the value of the magnetic flux angle, and the real part and imaginary part are used to determine twice the value of the sought magnetic flux angle using known complex calculation methods.
In this method, no mechanical sensor is required and the method is insensitive to uncertainties in the rotor resistance parameter, it being possible to dispense with voltage measurements. In addition, the supplying converter, which is present anyway, is used as a measurement signal generator.
International Patent Application WO92/19038 refers to the dissertation "Die permanenterregte umrichtergespeiste Synchronmaschine ohne Polradgeber als drehzahlgeregelter Antrieb," translated as "The permanently excited, convertor-fed synchronous machine without any rotor sensor as a regulated-speed drive", by H. Vogelmann (Karlsruhe University, Germany, 1986), which relates to a method for locating the rotor position. In this method, a relatively high-frequency current produced by means of a converter is superimposed as a test signal on the actual wanted signal. The basic idea in this case is that an electrical alternating signal which is locked in a certain (space vector) direction in general also causes a reaction in the orthogonal direction due to the different inductances in the longitudinal (direct) axis and transverse (quadrature) axis. Such a coupling is absent only when the alternating signal is applied exactly in the rotor longitudinal (direct) or transverse (quadrature) direction. A criterion results in this case as to whether the signal has or has not been applied in the desired particular direction. One precondition for achieving exact measurement results is a synchronous machine with permanent-magnet excitation and having a salient pole character, that is to say with unequal inductances in the longitudinal (direct) direction and transverse (quadrature) direction, as in the case of flux-concentrated arrangements, for example.
European Published Patent Application EP 0 228 535 A1 discloses a method and an apparatus for determining the flux angle in a rotating-field machine or for position-oriented operation of the machine. According to this method, a high-frequency element component is impressed on an electrical state variable of the stator winding system. In this case, the electrical state variables of the stator winding system are regarded as the currents and voltages in the individual stator coils, and the axes of the coils on which the high-frequency element (component) is impressed govern the direction of the impressed high-frequency element (component). It is self-evident that, for example, if a high-frequency current is impressed in one or more stator coils, high-frequency elements (components) also occur in the voltages of these coils and in the currents and voltages of the other coils. The voltage amplitude of these elements (components) depends on the angular difference between the flux axis and the direction of the impressed element. The amplitude of the high-frequency element (component) is thus determined from a state signal which reflects another state variable of the stator winding system; the sought angular direction is determined from the dependency of the detected amplitude on the predetermined direction of the impressed high-frequency part.
According to this method, a high-frequency element (component) must now be superimposed on the stator current by means of an additional nominal (command) vector whose frequency is higher than the frequency of the reference vector. The superimposition may be carried out by vector addition to the field-oriented nominal (command) vector or to the corresponding vector that is transferred to the stator-oriented coordinate system. Alternatively, the superimposition may be carried out in another, mathematically equivalent manner. As a result, the control vector also contains a correspondingly high-frequency element, which is impressed on the stator current via the converter. If the additional vector is a vector with a rapidly changing direction, then the control variable also changes correspondingly rapidly. The impression of a high-frequency element (component) in the stator current or in the stator voltage results in high-frequency voltage elements components (or current elements components) being coupled into the stator windings, whose envelope curves are associated with the position of the field axis or rotor axis. These envelope curves are associated, by suitable means, with the components of the direction vector. The direction vector is formed by means of a flux calculator which uses electrical machine variables to calculate the stator-related components of a model vector that describes the flux.
This method is based on the observation that a high-frequency element (component) of one state variable impressed in one of the stator coils induces a high-frequency element (component) in the other state variable of the same coil and high-frequency elements (components) in the state variables of the other coils. The high-frequency elements (components) depend on the position of the rotor or field axis. A sinusoidal oscillation at about 250 Hz is used as an additional nominal (command) vector. This method allows a synchronous machine to be operated in a field-oriented manner.
European Patent EP 0 071 847 B1 discloses a method and an apparatus for determining the rotor time constant of a field-oriented rotating-field machine. In this method, a disturbance variable whose profile is not constant with respect to time is added to the nominal (command) value of the field-forming (field-producing) component (parallel to the flux). The profile, with respect to time, of the actual value of an operating variable (torque or speed) is detected, and this operating variable is determined by the other component, that is, by the torque-forming (torque-producing) component of the machine current or the component of the machine current that exists perpendicular to the flux. This method further requires a correlation variable to be determined for correlation in the profile, with respect to time, of the disturbance variable and the detected operating variable. The parameter entered for the rotor time constant is varied as a function of the correlation variable until the correlation variable becomes virtually zero. The rotor acceleration or a rotating-field machine variable (torque or speed) derived from it is suitable for use as the operating variable to be detected. The disturbance variable preferably has a cyclic, statistical or pseudo-statistical profile with respect to time. This addition of a disturbance variable to the nominal (command) value for the field-forming (field-producing) current component has no effect on the torque-forming (torque-producing) component, provided the field orientation is exact. If the disturbance variable is chosen to have a profile with respect to time which differs sufficiently from the profile with respect to time of the nominal (command) current values (for example, the disturbance variable is provided with a profile that is at a considerably higher frequency), then the higher-frequency superimpositions caused by this can be associated unambiguously with the lack of decoupling of the flux and torque, that is, the incorrect setting of the rotor time constant. A sinusoidal signal is thus suitable for use as a disturbance variable.